Question

Explain all the models of atom........ i want full explaination of all the models.

Answer 1

JJ THOMSON'S MODEL OF ATOM

according to JJThomson,atom was considered as shpere of approx. 10-10 in which the positive charge in uniformly distributed. The electron were embedded in such a way through out the sphere to give maximum electrostatic attraction and maximum stability. Thomson's model was also called Plum pudding, Raisin pudding or watermelonmodel. this was discarded because it could not explain certain expt. resultas the scattering of alha particles by thin metal foil.

it failed to explain:

(1)how the positive and negative charges are arranged in an atom .

(2)there is no experimental proof of his atomic model.

RUTHERFORD'S MODEL OF ATOM

In an experiment with gold foil, Rutherford bombarded the gold foil withalpha particles. With Thomsons model as the basis, Rutherford expected small deviations; however, his findings were different from what was expected.

As we go further into this lesson, we will learn more about Rutherfords gold-foil experiment, his observations and his conclusions. We will also learn about the atomic model that he came up with on the basis of his conclusions.

Set up for Rutherfords experiment:

A thin gold foil, approximately 1000 atoms thick, was taken. Gold was chosen for its high ductility.
A detector screen with a small slit (for emission of radiation from the atom) was placed around the foil.
A source of alpha particles was kept in front of the foil.
The foil was bombarded with fast-moving alpha particles.
The set-up for Rutherfords gold-foil experiment is shown in the figure.



Rutherfords Expectations and Observations

What Rutherford expected?

Rutherford expected that the alpha particles would pass straight through the foil and only a small fraction of alpha particles would be deflected. This expectation was in compliance with Thomsons atomic model.

What Rutherford observed?



Rutherfords findings were contrary to his expectation. He observed that:

1. Most of the fast-moving alpha particles passed straight through the gold foil.

2. Some particles were deflected through the foil by small angles.

3. One out of every 12000 particles rebounded, i.e., they got deflected by an angle of 180.

What Rutherford Concluded from His Observations

Rutherford then carefully studied his observations and made the following conclusions.

Most alpha particles passed through the gold foil without any deflection. This indicates that most of the space inside an atom is empty.
Very few particles suffered a deflection from their path. This means that positive charge occupies very little space inside an atom.
Only a small fraction of particles underwent a180 deflection. This shows that the entire positive charge and mass of an atom are present within a very small volume inside the atom.

The major features of the Rutherford atomic model are as follows.

All protons are present inside the nucleus, which is situated at the centre of the atom.
Electrons reside outside the nucleus and revolve around the nucleus in well-defined orbits.
The size of the nucleus is very small in comparison to the size of an atom. As per Rutherford’s calculations, the size of the nucleus is 105 times smaller than an atom.
As the mass of the electron is negligible in comparison to the mass of the proton, almost all the mass of the atom is concentrated in the nucleus.

Drawbacks of Rutherford’s model are listed below:



Rutherford’s model failed to explain stability of atoms.
Rutherford’s model does not explain the distribution of electrons around the nucleus.
Rutherford’s model does not tell anything about the energy of electrons

The Bohr's Model of Atom can be described as follows :

An atom is made up of three particles : electrons, protons and neutrons. Electrons have negative charge, protons have positive charge whereas neutrons have no charge, they are neutral. Dueto the presence of equal number of negative electrons and positive protons, the atom on the whole is electrically neutral.
The protons and neutrons are locatted in a small nucleus at the centre of the atom. Due to the presence of protons, nucleus is positively charged.
The electrons revolve rapidly round the nucleus in fixed circular paths called energy levels or shells. The energy levels ar shells are represented in two ways : either by the number 1, 2, 3 , 4, 5 and 6 or by the letters K, L, M, N, O and P. The energy levels are counted from the centre outwards.

Answer 2

J j thomson atomic model:-jj Thomson in 1904, proposed that an atom was a sphere of positive electrically in which were embedded number of electrons , sufficient to neutralise the positive charge.this model is compared with water melon in which seeds are embedded, or with a cake or pudding in which raisins are embedded .that is why this model is called raisin pudding model

Rutherford atomic model:-Rutherford passed energetic alpha particle through a very thin (10^-7m) gold foil and concluded the following,
1) most of alpha particle (99%) passed through undeflected , concluding that most of the space in the atom are empty. 2) the rest of the alpha particle were deflected by small angles, and very few alpha particle refracted the Path (180) this concluded that the positive charge and whole mass of atom is concentrated at a small place called nucleus .
3) electron moves in circular path is called orbit and electron and nucleus are held together by electrostatic force of attraction.

Bohr's atomic model:-Bohr proposed the following postulate remove the drawback in Rutherford 's atomic model:
1) electron move around the nucleus in the circular path called orbit, each orbit having a particular amount of energy .the energy of orbit increases as the distance of orbit from nucleus increases .
2) an electron doesn't radiate energy as long as it revolves in a particular orbit.it makes a transition from one orbit to another , when emits or absorbs energy.
3) the angular momentum associated with an orbit is integral multiple of h/2pi.
e.g. mvr=nh/2pi.
4) during revolution in an orbit , the necessary centripital force is provided by the electrostatic force if attraction between electron and nucleus.
so,
mv^2/r=kZe^2/r^2

quantum mechanical model :- this is the most successful model of atom that describes the structure and configuration of atom in three dimensional space.
Erwin schrondinger modified Bohr's theory, who considered the position and momentum of electron at a time , by probability of finding position of electron at a time .
he achieve this by considering wave motion of electron in three dimensional spaces satisfying the following equation
d ^2₩/dx^2+d^2₩/dy^2+d^2₩/dz^2+8.pi^2.m/h^2 (E-V) ₩=0

where
₩ is wave function
h is plank constant
E is total energy of electron
V is potential energy of electron.